Colorless and pigmented oxidatively drying paints and coatings based on oxidatively drying oils, alkyd resins, epoxy esters and other oxidatively drying refined oils are known. These oils and binders crosslink oxidatively under the influence of oxygen (preferably atmospheric oxygen) by means of the addition of driers, such as metal carboxylates of transition metals. If this crosslinking takes place before the product is actually used, a solid barrier film, a skin, can form on the surface. Skin formation can occur in open or closed containers. This is highly undesirable and should therefore be avoided since it makes the paint more difficult to work with, and commonly interferes with the uniform distribution of the driers. The accumulation of the driers in the paint skin that forms can lead to considerable delays in the drying of the paint when it is applied.
Skinning of the paint film after application is also disadvantageous. Excessively rapid drying of the surface of the paint prevents the lower film layers from drying evenly because they are shielded from oxygen, which is prevented from sufficiently penetrating into and dispersing within the paint film. This can lead among other things to flow problems in the paint film, adhesion problems, or insufficiently hard films.
It is known to add organic substances to a paint that inhibit the reaction of the drier with (atmospheric) oxygen by binding the oxygen or by complexing of the drier metal.
U.S. Pat. No. 4,618,371 describes the use of aliphatic α-hydroxy ketones as anti-skinning agents. DE-A 1 519 103 discloses N,N-dialkylated hydroxylamines for this purpose. Because of their low volatility, however, hydroxylamines alone can lead to severe delays in drying and often also to reduced film hardness values, so that their possible applications are limited. They have not been able to gain commercial acceptance as anti-skinning agents. U.S. Pat. No. 6,730,157 describes the use of organic hydroxylamines such as diethylhydroxylamine and β-dicarbonyl compounds such as diethylformamide as anti-skinning agents. US patent application publication 2003/0047112 discloses a mixture of an aliphatic amine and/or its salt with a compound of the formula specified therein, such as diethyl formamide as an anti-skinning additive for lacquer systems. U.S. Pat. No. 6,224,659 discloses the use of a combination of tin compounds as anti-skinning agents for oxidatively drying binders.
A central issue in alkyd resin technology is to quickly cure, or dry the resin which occurs via oxidative crosslinking, while maintaining adequate anti-skinning properties. Oxidatively drying coatings typically include one or more “driers” such as metals to assist in the oxidative drying reaction. Combinations of a primary drier and a secondary drier are common. Cobalt is currently the most commonly used primary drier although other metals from Groups 1A, 2A, 3A, 4A, 5A, 1 B, 2B, 3B, 4B, 5B, 6B, 7B and 8B of the periodic table or combinations thereof can be employed.
Anti-skinning requires slowing the oxidative curing reaction at the air-resin interface while drying requires acceleration of the oxidative crosslinking throughout the resin film. Oximes, which act as oxygen scavengers, or suitable phenolic compounds are most often used as anti-skinning agents in industry. However, phenolic anti-skinning agents result in a significant delay in surface drying such that alone they are only suitable for certain coating compositions. Oximes such as e.g. methyl ethyl ketoxime (MEKO) or butyraldoxime, on the other hand, display only slight delays in surface drying due to their volatility. The high volatility of oximes results in rapid loss of this anti-skin agent from the alkyd in a storage can or applied film and thus does not adequately control skinning. The most significant disadvantage of the oximes, which are widely used today, lies in their toxicity. As a consequence of this, users have to observe elaborate personal protection precautions when working with paints containing oximes as anti-skinning agents.
It was discovered that the use of the combination of two or more oxygen scavengers as described below provides for inhibition of skinning with minimal impact on drying properties. In particular, the above-mentioned disadvantages of the specified hydroxylamines as anti-skinning agents could also be avoided by combining two or more oxygen scavengers, and hence products that better satisfy requirements as anti-skinning agents are obtained.
Incorporating a combination of two or more oxygen scavengers according to the present invention into an air-drying coating such as an alkyd resin coating provides an alkyd resin system which is resistant to undesirable skinning and exhibits improved drying of the resin films after application.